无限滚动视图 firebase 实时数据库错误

Question

im trying to implement an infinite scroll on my listview to load 10 posts more after scroll down and must hold the last 10 on screen because i dont have a button to back to the previous list, managed to retrieve the data, but when i try to scroll down to retrieve the next 10, it crash and give this error.

SearchRef = FirebaseDatabase.getInstance().getReference().child("Normas");


    if (user != null) {
        getThreads();
        threadsListView.setOnScrollListener(new AbsListView.OnScrollListener() {
            private int currentVisibleItemCount;
            private int currentScrollState;
            private int currentFirstVisibleItem;
            private int totalItem;
            private LinearLayout lBelow;


            @Override
            public void onScrollStateChanged (AbsListView view,int scrollState){
                // TODO Auto-generated method stub
                this.currentScrollState = scrollState;
                this.isScrollCompleted();
            }

            @Override
            public void onScroll (AbsListView view,int firstVisibleItem,
                                  int visibleItemCount, int totalItemCount){
                // TODO Auto-generated method stub
                this.currentFirstVisibleItem = firstVisibleItem;
                this.currentVisibleItemCount = visibleItemCount;
                this.totalItem = totalItemCount;


            }

            private void isScrollCompleted () {
                if (totalItem - currentFirstVisibleItem == currentVisibleItemCount
                        && this.currentScrollState == SCROLL_STATE_IDLE) {
                    /** To do code here*/
                    SearchRef.orderByKey().startAt(oldestPostId).limitToLast(10).addListenerForSingleValueEvent(new ValueEventListener() {
                        @Override
                        public void onDataChange(DataSnapshot dataSnapshot) {
                            for (DataSnapshot child : dataSnapshot.getChildren()) {
                                MessageThread messageThread = child.getValue(MessageThread.class);
                                if (messageThread != null) {
                                    oldestPostId = child.getKey();
                                    messageThread.thread_id = child.getKey();
                                    Log.d(TAG, "onDataChange: " + messageThread.toString());
                                }
                            }
                        }

                        @Override
                        public void onCancelled(DatabaseError databaseError) {

                        }
                    });

                }
            };

        });
    }



public void getThreads() {
    SearchRef.limitToLast(10).addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            Query ordenar = mDatabase.child("Normas").orderByChild("countthreads");
            threadsList.clear();
            for (DataSnapshot messageThreadSnapshot : dataSnapshot.getChildren()) {
                MessageThread messageThread = messageThreadSnapshot.getValue(MessageThread.class);
                if (messageThread != null) {
                    dataSnapshot.getChildrenCount();
                    messageThread.thread_id = messageThreadSnapshot.getKey();
                    Log.d(TAG, "onDataChange: " + messageThread.toString());
                }
                threadsList.add(messageThread);
            }
            threadsAdapter = new ThreadsAdapter(ThreadsActivity.this, R.layout.threads_listview, threadsList, ThreadsActivity.this, ordenar);
            Collections.reverse(threadsList);
            threadsListView.setAdapter(threadsAdapter);

        }
        @Override
        public void onCancelled(DatabaseError databaseError) {
            Toast.makeText(ThreadsActivity.this, "ThreadsActivity: " + databaseError.getMessage(), Toast.LENGTH_SHORT).show();
        }
    });
}

and this is the log after crash:

java.lang.IllegalArgumentException: You must use startAt(String value), endAt(String value) or equalTo(String value) in combination with orderByKey(). Other type of values or using the version with 2 parameters is not supported

Answer 1

My guess is that oldestPostId is null , which is not allowed.

You'll want to check for null before deciding to use oldestPostId :

Query query = SearchRef.orderByKey().limitToLast(10);
if (oldestPostId != null) {
  query = query.startAt(oldestPostId);
}
query.addListenerForSingleValueEvent(new ValueEventListener() {
  ...

Btw: Firebase Realtime Database now has a startAfter method , which means you won't be retrieving the same key twice.